Functional types of trees in temperate and boreal forests: classification and testing

Impacts of climatic change on ecosystems over large areas cannot usually be assessed on a species basis. The definition of plant functional types (PFTs) may make this task feasible, but its potential and limitations need to be evaluated carefully by quantitative tests. This paper presents such a method for testing a hypothesis about PFTs. A deductive approach is used to elaborate a functional classification of temperate and boreal trees based on their response to climatic change and their feedbacks to the climate system using four attributes: (1) evergreen/deciduous; (2) cold tolerance; (3) drought tolerance; (4) shade tolerance. These attributes are combined to yield six PFTs. The classification is tested by implementing the six PFTs in the forest gap model FORCLIM, and the behaviour of the PFT-based model is compared to species-based simulation results in Europe and in eastern North America. The six PFTs provide an accurate description of both the physiognomy and the composition of European forests in both transient and equilibrium phases. Under the IPCC 'BusinessAs-Usual' scenario of climatic change for southern and central Europe, some differences in the forest composition are simulated, pointing at the need of testing PFT schemes not only under current, but also under hypothesized future climates. For eastern North America it was necessary to estimate the parameters characterizing the six PFTs anew. When this was done, the PFT-based simulation yielded results paralleling those of the species-based model closely. The potential and limitations of the present classification are discussed. It is concluded that deficiencies in current gap models, but probably also different evolutionary pathways in North America as compared to Europe, are responsible for the different parameterizations on the two continents.